Characteristics of nuclease activity of the SbcCD complex from Deinococcus radiodurans

The bacterium Deinococcus radiodurans is extremely resistant to the intense ionizing irradiation which causes extensive DNA double-strand breaks (DSBs). The deinococcal SbcCD complex (drSbcCD) is required for DSB repair. The drSbcC and drSbcD genes were cloned and overexpressed in Escherichia coli cells, respectively. The nearly homogeneous drSbcC and drSbcD proteins were purified and reconstituted to form a stable complex in vitro. The drSbcCD complex has an ATP-independent 3'-->5' exonuclease activity to cleave both dsDNA and ssDNA substrates in the presence of either Mn(2+) or Mg(2+) ion. The drSbcCD complex also has an ATP-independent endonuclease activity. It can cleave the circular ssDNA, nick the supercoiled circular dsDNA, cleave the 3' flap DNA substrate at the site of the single-strand branch adjacent to duplex DNA, and cleave the hairpin DNA taking no account of the DNA end free or not. It is a kind of secondary structure-specific endonuclease. The drSbcCD complex still has a 3'-->5' exonuclease activity when the DNA termini are blocked by the proteins. These results suggest that the drSbcCD complex takes part in the metabolism of DNA, and its nuclease activities may play important roles in DNA repair process.